1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, in which an image developed with liquid developer and held on an image holding device is transferred onto a record medium.
2. Description of the Background Art
In an image forming apparatus such as a copying machine or a printer, image formation with liquid developer is generally performed as follows. A surface of an electrostatic latent image carrier such as a photosensitive member is charged, and image exposure based on image information is effected on the charged region to form an electrostatic latent image. The latent image thus formed is developed into a visible toner image with liquid developer, and the visible toner image is transferred onto a record medium such as a paper sheet.
For forming a color image with liquid developers, electrostatic latent images corresponding to various colors (e.g., cyan, magenta, yellow and black in the case of formation of a full color image) are successively formed on an electrostatic latent image carrier based on the image information. When the electrostatic latent image corresponding to one of the colors is formed, the visible toner image of the same color is then formed by developing the latent image with the liquid developer of the same color, and is transferred onto the record medium. In this manner, the transferred toner images of the respective colors are superposed on the record medium. The multi-layered toner image thus formed are generally fixed to a transferred image receiver under a heat and a pressure.
If fast formation of the color image is required, an image forming apparatus of a so-called tandem type may be employed. In the image forming apparatus of this tandem type, a plurality of units for forming and transferring visible toner images of different colors are successively arranged along a certain direction. The record medium is passed successively through visible image transfer portions in the above visible toner image forming and transferring units. A transporting member is employed for holding the record medium and successively passing the same through the transfer portions as described above.
In any kind of the wet image forming apparatuses using the liquid developer, a quantity of carrier liquid, which is adhered onto the image carrier together with the toner image, forms an important factor determining the result of the process for transferring the developed toner image formed on the electrostatic latent image carrier onto the record medium. More specifically, the following problems may arise depending on the quantity of the carrier liquid.
For example, the apparatus may employ transferring means which forms a transfer electric field for attracting the toner onto the transferred image receiver. In this case, the toner on the image carrier electrically migrates to the transferred image receiver through the carrier liquid coexisting with the toner. If the quantity of the carrier liquid is excessively small, the electrical migration (electrophoresis) and therefore the transfer cannot be performed sufficiently, resulting in lowering of an image density and/or variations in density. If the quantity of the carrier liquid is excessively large, the carrier liquid tends to overflow from the transfer portion, which is formed between the electrostatic latent image carrier and the transferred image receiver, and thereby may push out the toner on the electrostatic latent image carrier so that disturbance (i.e., a so-called image-flow) may occur in the image.
The transferred image receiver may be made of a liquid absorbent material such as paper. In this case, even if an appropriate quantity of carrier liquid coexisting with the toner image on the electrostatic latent image carrier is moved to the transfer portion, the carrier liquid in the transfer portion is absorbed into the transferred image receiver so that the quantity of the carrier liquid changes.
If the transferred image receiver is made of, e.g., MUCOAT 84 mg/m.sup.3 manufactured by Komine Co., Ltd., the appropriate quantity of the carrier liquid to be moved into the transfer portion is 0.1 mg/cm.sup.2. Meanwhile, if KOMINE 64 g/m.sup.3 manufactured by Komine Co., Ltd. is used, the required quantity of the carrier liquid to be moved into the transfer portion is 0.6 mg/cm.sup.2.
In addition to the liquid absorptance of the transferred image receiver, the appropriate quantity of the carrier liquid in the transfer portion also depends on a surface roughness of the transferred image receiver. The surface of a large roughness has many and/or large concavities, which must be filled with the carrier liquid for performing the electrical migration required for intended transfer.
In the color image forming apparatus of the foregoing tandem type, if the transferred image receiver is made of a liquid absorbent material, the transferred image receiver absorbs the carrier liquid during the process of transferring the toner image of a first color, and thereby may be completely or nearly saturated with the carrier liquid. In this case, the transferred image receiver absorbs no or substantially no carrier liquid in the process of transferring the toner images of the second and subsequent colors. Accordingly, the quantity of the carrier liquid of the first color, which is absorbed into the transferred image receiver, is different from those in operations of transferring the toner images of the second and subsequent colors, and the quantities of the carrier liquid must be controlled depending on the colors, respectively. Further, a similar situation also occurs if the transferred image receiver has a large surface roughness. In this case, the quantity of the carrier liquid must be enough to fill the concavities on the surface of the transferred image receiver in the transfer operation for the first color, and the appropriate quantities of the carrier liquid for the second and subsequent colors are small because the concavities on the surface of the transferred image receiver are already filled with the carrier liquid.
In view of the above, the quantity of the carrier liquid moved to the transfer portion must be controlled depending on the kind of the transferred image receiver.
Japanese Laid-Open Patent Publication No. 8-297418 (297418/1996) has disclosed a manner for moving an appropriate quantity of carrier liquid to the transfer portion. In the disclosed manner, surplus carrier liquid is removed by squeezing means after an electrostatic latent image on an electrostatic latent image carrier was developed into a visible toner image, and conditions for squeezing and removing the carrier liquid are controlled in accordance with the kind of the transferred image receiver. For example, in the structure employing a squeeze roller, a space between the squeeze roller and a photosensitive member is controlled and/or a peripheral speed of the squeeze roller with respect to the photosensitive member is controlled. In the structure employing a squeeze charger, a voltage applied to the squeeze charger is controlled to change the quantity of carrier liquid which remains after squeezing.
In an image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 8-152788 (152788/1996), control is performed to determine a quantity of prewet liquid, which is applied onto a photosensitive member prior to development, in accordance with the kind of the transferred image receiver.
However, the image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 8-297418 requires a complicated structure which can change the squeezing conditions for removing the surplus carrier liquid by the squeezing means. If a large quantity of carrier liquid is required depending on a kind of the transferred image receiver, the control is performed to increase the quantity of the residual carrier liquid, which is not removed by squeezing and coexists with the toner on the photosensitive member. However, the residual carrier liquid contains many ions of the polarity opposite to the chargeable polarity of the toner. Thereby, the ions of the opposite polarity near the toner increase in quantity in accordance with increase in quantity of the carrier liquid. If the ions of the polarity opposite to the toner chargeable polarity are collected near the toner, the toner behaves as if the quantity of its charges lowers, and the electrophoresis of the toner in the transfer electric field formed in the transfer portion cannot sufficiently occur. Thereby, variations in image density are liable to occur in the toner image when it is transferred onto the transferred image receiver.
In the color image forming apparatus of the tandem type described above, the quantities of the residual carrier liquid must be controlled in view of the order in which the toner images of the respective colors are transferred, in addition to the control of the quantities of the residual carrier liquid under the conditions depending on the kinds of the transferred image receiver. Therefore, complicated control is required, resulting in increase in manufacturing cost of the image forming apparatus.
Further, in the image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 8-152788, the prewet liquid is applied onto the photosensitive member prior to the development. Therefore, the electrostatic latent image on the photosensitive member and a developing electrode in the region for developing the electrostatic latent image are spaced by a large distance, or the latent image and the toner are spaced by a large distance so that the developing speed decreases, and fast image formation cannot be performed. Further, such a problem arises that the prewet liquid is mixed into the liquid developer.